A color liquid display device contains a liquid crystal and a color filter plate as its constitutional components for controlling the transmission or reflection of light. The color filter plate, which contains red, green and blue color filters, and is prepared by forming red-, green-, and blue-colored image elements and a black matrix on a conductive glass substrate, is one of the most expensive components of the color liquid display displace. The reasons for its high cost can be attributed, at least in part, to the complicated manufacturing process involved, the high raw material cost, and the low utilization rate of the raw material.
Currently, at least four methods have been taught in the prior art for making color filters. These include: (1) dyeing, (2) printing, (3) pigment dispersion, and (4) electrodeposition,. Among these methods, the pigment dispersion method is believed to be the most widely-used method. However, the pigment dispersion method suffers the problems of high manufacturing cost and low raw material utilization rate as discussed described above. The electrodeposition method has been recognized as being capable of resulting in lowered cost; however, it suffered from other problems, most notably the incompatibility problems that have been observed between the raw materials used in the process. For example, when an anionic electrodeposition resin is paired with a positive photoresist to coat on the entire surface of an ITO glass, the anionic resin is often attacked by the developer solution, which is used to dissolve the photo-exposed positive photoresist. On the other hand, if a cationic electrodeposition resin is paired with a positive photoresist, another problem associated with the blackening of the ITO glass would arise. Some of these problems may be ameliorated by etching the ITO glass into a stripe pattern (i.e., containing a plurality of isolated and elongated stripes), prior to connecting the ITO glass to the electrodes to proceed with the electrodeposition process. However, this technique creates difficulties in forming the black matrix, which contains a rectangular frame and other line boundaries to improve the sharpness of the color filters.
In U.S. Pat. No. 4,820,619, the content thereof is incorporated herein by reference, a photosensitive composition is disclosed for use in preparing a color filter which contains a copolymer of glycidyl (meth)acrylate or glycidyl (.alpha.-methyl)vinyl ether with a (meth)acrylic amide or ester having a quaternary ammonium salt structure, and an aromatic azide as a photosensitizer. U.S. Pat. No. 4,837,098, the content thereof is incorporated herein by reference, discloses a colored filter layer comprises three groups of filter picture elements having spectral characteristics respectively corresponding to red, green and blue. Each group of filter picture elements (R, G, B) are made of polyimide resin and dye contained therein. Because of the relatively inadequate light and heat resistances of the dyeing materials, the dyeing method has been pretty much discarded by the manufacturers.
The printing method uses pigment as the raw material, thus it does not experience the heat resistance problem experienced by the dye rag method. However, the relatively poor resolution of the printing method, which is typically coarser than 100-.mu.m, does not satisfy the needs of most manufacturers. The pigment dispersion method can be classified into two major types: etching method and photo-imaging method. The etching method has the advantage that it utilizes color dyes that exhibit improved heat-resisting characteristic. However, because of the relatively complicated process involved, and the somewhat excessive raw material requirement, the etching method has gradually lost its favor. The photo-imaging method utilizes a photosensitive resin, and is the most popular method at the present time. U.S. Pat. No. 5,085,973 and Japan Patent 2-208602, the contents thereof are incorporated herein by reference, disclose the steps and materials generally involved in the photo-imaging process. Because of its high resolution, better than 10-.mu.m, and the high heat resistance of its constituent coloring material, the photo-imaging process has been widely accepted by the industry. One of the drawbacks of the photo-imaging process, however, is that large amounts of the pigments, typically more than 90%, are wasted. Thus, the photo-imaging process was significantly more expensive than other processes. It has been attempted to employ other more precise coating methods, such as roller coating method, to replace the traditional spin coating method, so as to reduce the amount of the expensive pigments that because waste. However, these coating methods do not provide enough flatness, and the attempts were largely unsuccessful.
In accordance with the shape of the transparent conductive substrate (which is typically comprised of indium-tin oxide, or ITO), the electrodeposition coating process can be classified into two main types: "ITO with pattern" (or the "patterned ITO" process) and "ITO without pattern" (or the "patternless ITO" process). It should be noted that the term ITO is used here to represent, in a very broad sense, the transparent conductive substrate, because of the prevalent use of the indium-tin oxide material for forming the transparent conductive substrate. Of course, an "ITO", according to this description, can be made of other transparent conductive materials.
The patterned ITO process was developed earlier than the patternless ITO process. U.S. Pat. No. 4,812,387, the content thereof is incorporated herein by reference, describes an example of the electrodeposition coating process, by which a a light-shielding electrodepositing coating composition is first formed in a transparent electrically conductive circuit pattern so as to suppress the reaction of a photo-setting material against light which is to be applied to the electrodeposition coating. With the electrodeposition coating processes, a transparent electrode is prepared by patterning a transparent electrically conductive film (typically an indium-tin oxide, or ITO) which is deposited on a substrate and serves as an electrode, and an electric voltage is applied only to a portion of the patterned transparent electrode which is to be dyed in the same color. The substrate is then immersed in a coloring electrodeposition bath (i.e., an electrolyzing bath) containing appropriate polymers and pigment dispersed in water, and a colored layer is then formed by electrodeposition. This completes the formation of the first color filter (or the first electrocoat). Thereafter, electric voltage is similarly applied only to another portion of the substrate which is to be dyed with a different color, and the substrate is then immersed in another colored electrodeposition bath for forming a different color layer via electrodeposition. This procedure is repeated until all the desired colored layers are formed. In this process, because the colorants are pigments, the color filters so made exhibit high heat resistance. Furthermore, because the conductive layer is formed using a photo-imaging procedure, resolution can be better than 10-.mu.m. Another distinct advantage of the electrodeposition process over the pigment dispersion process, is that the utilization rate of the pigments can be as high as 98%, thus it presents the advantage of having superior cost-effectiveness than the pigment dispersion process. However, in the electrodeposition process, because the conductive layer having the same color pixels must be contiguous, the color pixels are typically arranged in straight lines (i.e., a stripe pattern). Thus this process lacks the freedom enjoyed by other processes, which allow the color pixels to be arranged generally in any desired pattern. Furthermore, because the color pixels are arranged in stripes of straight lines, additional steps are required to form the black matrix. Japan Patent 61-203403 discloses a multiple photo-imaging process for forming the black matrix. This results in increased production cost. The electrodeposition process is also described in, for example, U.S. Pat. Nos. 4,781,444, 5,206,750, and 4,617,094, the contents thereof are incorporated herein by reference.
With the patternless ITO method, which does not require the etching of an ITO pattern, the conductive ITO layer is first coated with a photoresist layer. After exposure and development, a conductive ITO layer having exposed areas corresponding to the intended positions of pixels of the same color is developed. The undeveloped photoresist serves as an insulation layer. A color pigment is then electrodeposited onto the exposed ITO layer to form a color layer. This procedure is repeated until all the red, blue, and green color pixels are coated, via electrodeposition, on the ITO layer. This method retains the main advantage of the electrodeposition method, in that it provides high pigment utilization rate, while allowing the pattern and arrangements of the color pixels to enjoy a high degree of design flexibility. There are, however, several drawbacks with the patternless ITO method which are difficult to overcome. For example, because the patternless ITO method uses the photoresist as the insulation layer, the coloring resins are subject to attacks by the developer solution when anionic coloring resins are used. While the resistance (with respect to the basic developer solution) of the coloring resin can be improved by increasing the hardening temperature (to above 120.degree. C.), most positive photoresists (which typically belong to the quinodiazide type) will encounter a thermal crosslinking reaction when temperature reached 120.degree. C., thus making it difficult to remove the exposed photoresist. To overcome this problem, U.S. Pat. No. 5,439,582 and Japan Patent No. 5-93807 disclosed methods which utilized low-temperature reactive electrodepositing resins as well as other more expensive positive photoresists. The low-temperature reactive electrodepositing resins, however, caused other problems. For example, it reacted with the electrodepositing tank fluid, and thus shortened the useful life thereof. Additionally, there is very limited selection and supply of other commercially available positive photoresists. Because of these problems, artionic electrodeposition method is seldom used in the industry.
A number of transfer printing techniques have been taught in the art to prevent the attack on the anionic deposited resin by the developer solution. These include the methods disclosed in U.S. Pat. Nos. 4,853,092, 5,314,770, 5,385,795, and 4,902592, Japan Patent No. 5-164913, and European Pat. No. 0299508, etc. The contents of these teachings are incorporated herein by reference. However, the transfer printing techniques often result in inadequate precision and adversely affect the yield rate. Another method to avoid the attack by the developer solution is to use cationic electrodepositing coloring resins. This method is taught in, for example, U.S. Pat. No. 5,214,542, Japan Patent 4-324801, and European Pat. 0587033, the contents of these teachings are incorporated herein by reference.. However, the cationic resins often caused the ITO layer to be subject to a reduction reaction and become darkened. This causes the light transmittance to be greatly reduced.
U.S. Pat. No. 5,186,801 discloses a method by which the electrodepositing resin was formulated into a photosensitive resin which will harden upon exposure to light. This modification may eliminate the problem associated with the attach by the developer solution; however, the resultant photoresitive resin often provides relatively poor resolution; this greatly diminishes the effectiveness, thus the commercial applicability, of this method.